Reference is made to FIG. 1 showing a conventional time-of-flight (ToF) type optical ranging system 10. The system 10 includes a support substrate 12. Mounted to a top surface of the support substrate 12 is an electromagnetic radiation emitter 14. The emitter 14 may, for example, comprise a vertical cavity surface-emitting laser (VCSEL) device. The emitter 14 emits a beam 16 of electromagnetic radiation at a desired wavelength (or range or wavelengths) that is operable to illuminate a target with an emission field of view (FOV) 18. Further mounted to the support substrate 12 is an integrated circuit sensor 20. The sensor 20 may, for example, comprise a photosensing region 22 including one or more single photo avalanche diodes (SPADs) which function as photosensitive elements (pixels) to detect the electromagnetic radiation that has been emitted from the emitter 14 in beam 16 and returned after being reflected by said target. To increase sensing resolution, the photosensing region 22 of the integrated circuit sensor 20 may include a plurality of SPADs arranged in an array format. The photosensing region 22 has a detection field of view (FOV) 24 which overlaps the emission field of view 18.
A body 26 of opaque encapsulating material (for example, a resin material) is provided on the support substrate in a position which protects the electromagnetic radiation emitter 14 and the integrated circuit sensor 20. A first opening 28 in the body 26 is provided at the location of the electromagnetic radiation emitter 14 to permit emission of the beam 16 towards the target. The first opening 28 may be closed or filled by a material which is transparent at the desired wavelength (or range or wavelengths) for the electromagnetic radiation emitted from the emitter 14. A second opening 30 in the body 26 is provided at the location of the photosensing region 22 facing the target. The second opening 30 may also be closed or filled by a material which is transparent at the desired wavelength (or range or wavelengths) for the electromagnetic radiation emitted from the emitter 14.
Operation of the system 10 is constrained from an optical perspective by two competing factors. The first factor is referred to as “absolute signal” and refers to a certain number of photons emitted from the emitter 14 which must be received (after target reflection) by the photosensing region 22 in a certain time period in order to achieve a desired characteristic of target detection (speed, accuracy, etc.). The second factor is referred to as “signal to noise” and refers to the relationship (for example, in the form of a ratio) between the reception of reflected radiation (the signal) and the reception of ambient light (the noise) which can degrade system performance. In designing the optics and operation of the ToF system, there is a need to balance these competing factors. Still further, there is a need to extend the range of target detection.